With a view to reducing energy consumption, it is now becoming common to stop the thermal engine of a vehicle when it comes to a halt, and to restart the engine motor to move off again, in a way which is automatic and totally transparent for the driver.
This is possible due to the development of alternator-starters, which have been rendered highly efficient by the use of electronic power systems controlled by circuits which rely on digital techniques, mainly based on the use of microprocessors or microcontrollers.
One example of such techniques is provided in the document U.S. Pat. No. 5,317,248, which describes the generation by a microcontroller system of the PWM switching pulses of the phase currents of a brushless electric motor.
The microcontroller utilised comprises both a central processing unit and programmable delay circuits enabling waveform synthesis, known as PWM (Pulse Width Modulation), of the switching signals.
Another example of application of digital techniques is given in the document US20040108840, which describes the voltage regulation of an alternator, or generator, by means of a microcontroller driving the duty cycle of the periodic excitation current of the machine on the basis of sampled values of the DC voltage generated.
The response time of this regulator to a load variation seems to be several seconds, or even tens of seconds.
As modern vehicles use ever more numerous and sophisticated electrical equipment, it is highly desirable for the device for adjusting the onboard power supply to a load variation to have a much lower response time, in order to avoid the phenomenon of “hunting” and vibrations.
When an alternator or a reversible machine is regulated by a digital system (microcontroller, microprocessor or programmable logic), the principle of regulation introduces a phase delay in the servo-control system linked to the cycle time and to the calculation delay.
So if T is the cycle time and Tc the calculation delay defined by the time which separates the quantity to be controlled (in particular the battery voltage, but also the excitation current, for example).
The phase delay θ(f) in degrees at a frequency f is expressed approximately by the expression:θ(f)=(T/2+Tc)*f*360°
In a traditional digital regulator, the control cycle time T is equal to the excitation period Te. The regulation loop refreshes the value of the duty cycle once per excitation cycle. This delay in the regulation loop is thus strongly linked to the switching period of the power transistor, as the calculation delay can generally be defined at the design stage to stay as low as possible, and small with respect to Te/2.
There is consequently a practical limitation of the regulation bandwidth to about Fe/8 to Fe/10, where Fe is the excitation frequency 1/Te.
The excitation frequency is specifically selected as a function of the power dissipated by the transistor during the switching phases. This dissipated power leads to a rise in the temperature of the chip, which is itself limited by the maximum junction temperature permitted by the technology utilised. This frequency thus cannot be increased without giving rise to new temperature constraints on the switching component.